The present invention is directed to an improved automobile tire tread pattern. Such a tread pattern extends circumferentially around the tire and axially across the ground contacting portion of the tire from one sidewall to the other. The thick elastomer tread band, has variously disposed grooves made in such a way as to subdivide said band into ribs and/or blocks that are separated one from the other by a plurality of grooves. Said ribs and said blocks are generally also provided with sipes, i.e., with narrow cuts, directed radially inwardly from the surface of the tread band, which have a variable depth and which can also connect to the sides of the ribs and the blocks.
In fact, the whole assembly of grooves and sipes constituting the tread pattern form a characteristic and distinctive element of the tire and which is variable, depending upon the type of usage to which the tire is subjected.
For example, those tires that require good non-skidding characteristics because they are used on snow covered or muddy ground or on rugged terrain, present tread patterns that are heavily "blocked" with wide and deep cut-outs and grooves and by rather massive blocks; while tires that are usually for use on smooth roads are normally distinguishable by their big circumferential grooves, usually having a zig-zag configuration, from which branch out very narrow transversal sipes which, at times, extend axially over the entire rib width and generally have a tortuous configuration. In fact, the main purpose of these tread patterns is to break the liquid film that is created between tire and road, under wet road conditions, with thus minimizing the dangerous "aquaplane phenomena", while obtaining a high degree of steering control, driving stability and road holding behavior.
For those tires destined for the sports class type of cars, i.e., tires for usage on roads that are generally in a good condition but which have to undergo critical use conditions owing to the great acceleration and high speeds involved, even in cornering, it has recently been found quite convenient to employ tread patterns having a plurality of blocks, disposed in circumferentially extending rows, that are axially separated one from the other by circumferential rectilineal grooves and by obliquely disposed transversal grooves, in such a way that the blocks have a substantially rhomboidal form, i.e., the opposite sides are substantially parallel to each other, and the two adjacent sides, of substantially different lengths, are inclined with respect to each other at an angle other than 90.degree..
In the prior art tread patterns, the transversal grooves that, along with the longitudinal grooves defined in the blocks, had an inclination which was contrary (with respect to the circumferential direction of the tire) to that of the adjacent rows. Successively, these grooves were disposed so as to all be oriented in the same sense with respect to said circumferential direction. Nevertheless, in the more recent tires, it proved to be more convenient to dispose said transversal grooves in a herring-bone pattern, i.e., inclined in such a manner as to have them all converging in the same direction towards the equatorial plane of the tire. Such a tire has the inclination of these grooves in the opposite sense to one another, with regard to the tire's circumference, in the two tread band portions flanking said equatorial plane.
Hence, a series of changes have been made in the tread pattern, finally evolving to the so-called "directional" type of tread, i.e., a pattern substantially symmetrical with respect to the equatorial plane, but asymmetrical with respect to any whatsoever axial plane, for which reason the tire's contact area upon the ground changes upon reversing the sense of rotation of the wheel.
Therefore, said tires have a preferential sense of rotation that is usually indicated by stamping a special reference mark, an arrow for example, on the tire's sidewall.
Owing to this preferential direction of rotation, these tires can give a positive or negative performance in terms of their directional stability and their resistance to the aquaplane phenomena, depending upon their direction of rotation.
Apart from this, owing to the increased demands for a better performance from this type of tire and this type of car, the traditional tread patterns have also shown to be strongly sensitive to any sort of use that only involves extreme conditions of usage and which just involves those blocks disposed on one side of the equatorial plane and just a certain portion of each block, with compromising, in said manner, a uniform wearing pattern, and hence, the service life of the tire itself and its reliable performance throughout that life.